It isn't quite that chaotic. The attorneys will negotiate and work with the judge to figure out how to instruct the jury. And before the jury goes to deliberate on the decision, they are given fairly detailed instructions on what the law is, what evidence to consider, and sometimes what to specifically not consider.
As the other posters said, most jurisdictions follow specific recommendations on avoiding telling the jury to do whatever they want. And different jurisdictions have different rules on whether or not the jury can truly redefine the law or facts. Some places have that ingrained in their state constitution, but most do not. And civil vs. criminals trials also have different rules. And where I live, there are even different rules for specific types of civil trials.
It really is all fairly complex, hence every trial will give a specific set of instructions to that jury. And that is also why an attorney in one jurisdiction might not be correct on how the details work in another one. (Although the gist of it all will be the same.)
At the same time, this is also why we don't know for sure what the jury said or thought. We can get all their instructions - those are public record. But the actual deliberation is not.
As always, IANAL. Other commenters already seem to be annoyed at my layman's take on it all, but this is what I know from being a law school dropout and having served on juries.